Thermal and catalytic conversion of hydrocarbons to olefins is an important industrial process producing billions of pounds of olefins each year. Because of the large volume of production, small improvements in operating efficiency translate into significant profits. Catalysts play an important role in more selective conversion of hydrocarbons to olefins.
Particularly important catalysts are found among the natural and synthetic zeolites. Zeolites are complex crystalline aluminosilicates which form a network of AlO.sub.4 and SiO.sub.4 tetrahedra linked by shared oxygen atoms. The negative charge of the tetrahedra is balanced by the inclusion of protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes. The ability of zeolites to adsorb materials also enables them to be used in catalysis. There are a large number of both natural and synthetic zeolitic structures. The wide breadth of such numbers may be understood by considering the work "Atlas of Zeolite Structure Types" by W. M. Meier, D. H. Olson and C. Baerlocher (4th ed., Butterworths/Intl. Zeolite Assoc. [1996]). Catalysts containing zeolites have been found to be active in cracking light naphtha to ethylene and propylene, the prime olefins.
Of particular interest are the acidified zeolites effective for conversion of light hydrocarbons such as low boiling naphthas to the prime olefins. Typical catalysts include ZSM-5 zeolite described and claimed in U.S. Pat. No. 3,702,886, and ZSM-11 described in U.S. Pat. No. 3,709,979, and the numerous variations on these catalysts disclosed and claimed in later patents.
Previous uses of multiple reaction or temperature zones in prime olefin production have used hydrocracking or hydrogenolysis to produce ethane and propane with little production of prime olefins in the first stages. Franck et al., U.S. Pat. No. 4,137,147 used multiple hydrogenolysis stages to which each stage operated at 5.degree. C. to 25.degree. C. higher than the preceding stage. The light hydrocarbons up to C.sub.3 in the effluent from the hydrogenolysis stages were then steam cracked to prime olefins, while the C.sub.4 + production was separated and at least part of it sent to further hydrogenolysis for additional ethane and propane production. The steam cracking unit was supplied a fraction consisting essentially of ethane and propane for conversion to ethylene. Lionetti et al., U.S. Pat. No. 4,388,175, discloses a two stage system for production of aromatics from heavy oil. The second stage is operated at a higher temperature than the first to produce light naphtha, gasoline and needle coke. There was no indication of any application to prime olefins production. Tabak, U.S. Pat. No. 4,487,985 and its divisional U.S. Pat. No. 4,560,536 teaches oligomerization of lower olefins in a multistage series of reactors wherein catalyst partially inactivated in the primary stage is employed at a higher temperature in a secondary stage prior to catalyst regeneration. In European patent application 0 023 802 a hydrocracking step produces C.sub.2 to C.sub.5 alkanes from which prime olefins are produced by downstream thermal cracking at a higher temperature. GB 2,105,362 teaches a two stage thermal cracking process in a catalyst free system wherein the first reaction zone heats the steam/feedstock from 800.degree. C. to 1000.degree. C. and then passes the feedstock to a second catalyst free zone where it is heated from 850.degree. C. to 1150.degree. C. Mauleon et al., U.S. Pat. Nos. 5,506,365 and 5,264,115, teach a multiple zone process wherein hot catalyst is used in a mild steam thermal cracking process and reacted further downstream with additional catalyst at a lower temperature in a process aimed at gasoline production.
European Patent Application 0 262 049 teaches steam cracking of hydrocarbons (propane is exemplified) followed by contact with a multi-component zeolite-containing catalyst. The thermal cracking unit is operated at a higher temperature than the downstream catalytic cracker. Adams, U.S. Pat. No. 3,360,587, also teaches a steam cracking step followed by a catalytic cracker, again the catalytic cracker is at a lower temperature than the upstream thermal cracker. In European patent application 0 023 802 a catalytic (hydrocracking) reaction stage produces mainly C.sub.2 to C.sub.5 paraffins which are subsequently fed to an optional higher temperature thermal cracking unit for conversion to prime olefins. Published PCT application WO 95/13255 describes an integrated system wherein a light fraction is separated from the effluent of a deep catalytic cracking unit running on a relatively heavy oil fraction and recycled to a thermal cracking unit to produce prime olefins. Published PCT application WO 86/02376 discloses heavy oil cracking including a pre-pyrolysis cracking step followed by separation of an overhead stream that is thermally cracked for prime olefin production. Burich, U.S. Pat. No. 3,702,292, discloses an integrated refinery apparatus wherein various streams are separated and fed to both a hydrocracking unit and a thermal cracking unit. Derwent WPI Accession No. 88-053890/08 for Japanese patent 60235890 discloses thermal cracking of hydrocarbons in a two stage system. Derwent WPI Accession No. 86-011144/02 for Japanese patent 63010693 discloses feeding by-product light oil containing olefins from a catalytic cracking unit to a thermal cracking furnace with prime olefins recovered in high purity.
Heretofore the art has not recognized that cracking an olefin mixture over suitable catalysts followed by thermal cracking can result in significant increases in production of prime olefins without prior separation of components or removal of C.sub.4 + materials from the feed stream.